In all vertebrates and most invertebrates, the brain is the organ that acts as the center of the nervous system. It is located in the head, usually close to sensory organs, such as vision. It is the most complex organ in vertebrates. In humans, the cerebral cortex contains about 14 – 16 billion neurons, and the number of neurons in the cerebellum is estimated to be 5 – 70 billion. Each neuron connects to thousands of other neurons through synapses. These neurons communicate with each other through long protoplasmic fibers called axons, which transmit a series of signal impulses called action potentials to the brain or body far away from a specific receptor cell.

Physiologically, the brain exerts centralized control over other organs of the human body. They act on the rest of the body by producing patterns of muscle activity and by driving the secretion of chemicals called hormones. This kind of centralized control can make rapid and coordinated response to environmental changes. Some basic types of reactivity (such as reflexes) can be mediated by the spinal cord or peripheral ganglia, but complex and purposeful control of behavior based on complex sensory inputs requires a concentration of the brain's information integration capabilities.

Now we have a fairly detailed understanding of the operation of single brain cells, but the way they work together in thousands of collections remains to be solved. The latest model of modern neuroscience regards the brain as a biological computer, whose mechanism is quite different from that of electronic computer, but in a sense, it is similar, that is, it obtains information from the surrounding world, stores and processes information in various ways.

In this paper, the brain characteristics of the whole animal species are compared, of which vertebrates are the most concerned. It shares other brain properties with the human brain. The article on human brain introduces the different ways of human brain and other brains. Instead, there may be several topics covered here, because there are more topics to talk about in the human environment. The most important is the effects of brain diseases and brain damage described in human brain articles.

The shape and size of the brain vary greatly between species, and it is often difficult to identify common features. However, there are many brain structural principles that apply to many species. Some aspects of brain structure are almost common to all animal species. Others separate the "advanced" brain from the more primitive, or the vertebrate from the invertebrate.

The easiest way to get information about brain anatomy is through visual inspection, but many more complex techniques have been developed. Brain tissue in its natural state is too soft to use, but it can be hardened by immersion in alcohol or other fixatives and then cut open to examine the interior. Visually, the interior of the brain is divided by areas of so-called gray matter with dark colors and areas of white matter with lighter colors. More information can be obtained by staining brain sections with various chemicals, which bring out areas with high concentrations of specific types of molecules. Microscopes can also be used to examine the microstructure of brain tissue and track how connections from one brain region to another are made.

Information from the sensory organs is collected in the brain. There it is used to determine what action the organism will take. The brain processes raw data to extract information about the structure of the environment. Next, it combines the processed information with information about the animal's current needs and memory of past situations. Finally, based on the results, it generates the motor response mode. These signal processing tasks require complex interactions among various functional subsystems.

The function of the brain is to provide consistent control over animal behavior. A concentrated brain enables muscle groups to co activate in a complex way. It also allows the stimulation of impact on one part of the body to cause reactions in other parts, and can prevent cross interaction between different parts of the body.